4 Drive Synchronization Using BEMF

As the ZCD only happens for one phase at a time, there is no need to sample all three phases simultaneously. Thus, the required components to sample the BEMF can be given by either three comparators, or one comparator with a multiplexed input selection, as shown in Figure 4-1.

The comparator delays and MUX stabilization must be taken into consideration. To avoid delays, a priority selection method can be used: Measure the first phase of a motor until a zero cross is reached, then measure the second phase until another zero cross is reached, and so on.

Figure 4-1. Analog Comparator Possible Configurations

Based on the values of the sampled BEMF, the motor can be ahead or behind the drive field. To reduce the error between the drive field and the motor, a control algorithm can be introduced to improve the synchronization of the motor commutation. Figure 4-2 shows a rendering of all three drive channels with all three filtered BEMF channels when the motor can be behind the drive field.

Figure 4-2. Motor’s Phases and Measured BEMF for each Phase
Based on the information obtained from the BEMF ZCD, there are three possible cases:
  1. The BEMF Signal is ahead of the Drive Signal, so the Drive speed must be reduced to keep synchronization.
  2. The BEMF Signal is behind of the Drive Signal, so the Drive speed must be increased to keep synchronization.
  3. The BEMF Signal and Drive Signal are in phase with one another, within a range.

Figures 4-3 and 4-4 show 1 and 2 for one motor phase and its BEMF signal. The first figure shows the case when BEMF is ahead of Drive. The second figure shows the case when BEMF is behind the Drive.

Figure 4-3. BEMF Signal Ahead of Sine Drive Signal
Figure 4-4. BEMF Signal Behind of Sine Drive Signal